The present invention is related to digital processing, and more particularly, to digital demodulation devices and methods.
Please refer to FIG. 1. FIG. 1 illustrates a mathematical model of a digital demodulator in a receiver according to the related art. As shown in FIG. 1, mixers 10-1 and 10-2, filters 20-1 and 20-2, and an absolute value calculation unit 40 (which is labeled as “ABS”) represent digital operations of mixing, filtering, and absolute value calculation, respectively. In addition, adders 30-1 and 30-2 represent involving quantization noises e1(t) and e2(t) during digital operations of the upper and the lower paths shown in FIG. 1, respectively. Additionally, an amplitude modulation (AM) signal (S(t)*cos(2*pi*fc*t)) inputted into the mixers 10-1 and 10-2 represents a carrier cos(2*pi*fc*t) carrying S(t) (which is a function of time t), where the AM signal's envelope corresponds to S(t), pi (i.e. π) represents the ratio of the circumference of a circle to its diameter, and fc represents the frequency of the carrier cos(2*pi*fc*t).
In an ideal case, the mixer 10-1 mixes the AM signal (S(t)*cos(2*pi*fc*t)) with a signal cos(2*pi*fc*t), and the mixer 10-2 mixes the AM signal (S(t)*cos(2*pi*fc*t)) with a signal sin(2*pi*fc*t), so the digital demodulator may correctly operate to generate a demodulated result (S(t)+e3(t)), where e3(t) represents random noises from the quantization noises e1(t) and e2(t). In a real case, however, the frequency fc of the signal cos(2*pi*fc*t) and the signal sin(2*pi*fc*t) respectively utilized by the mixers 10-1 and 10-2 in this mathematical model is typically inaccurate due to difficulties in design. For example, fc′=fc+50 Hz, which means there exists a difference of 50 Hz between the inaccurate frequency fc′ and the carrier frequency fc generated in a transmitter which transmits the AM signal (S(t)*cos(2*pi*fc*t)) to the receiver. As a result, the demodulated result (S(t)+e3(t)) is overlapped with a noise component of 50 Hz. Unfortunately, if S(t) represents an audio signal to be played back, the noise component of 50 Hz will be heard and can be very irritating to a listener.
Typically, the related art method involves plenty of calculations which must be applied to correct the inaccurate frequency fc′, so as to accurately lock onto the carrier frequency fc generated in the transmitter. Therefore, cumbersome calculations lead to a heavy processing load for a digital processing unit of the receiver. In addition, it would be very complicated to implement the corresponding algorithm in order to achieve better performance.